1. Field of the Invention
The present invention relates to a surface acoustic wave device, and, more specifically, to a surface acoustic wave device having low loss and few ripples in in-band frequency characteristics.
2. Description of the Prior Art
In the conventional method for making an excellent surface acoustic wave device having low loss and excellent out-band frequency characteristics, patterns made of a metal thin film are formed outside input and output transducers and grounded. Such a method is disclosed, for example, in CPM 81-20 of Vol. 81 No. 78 of Denshi Buhin and Zairyo (electronic parts and materials), a technical research report issued by the Institute of Electronics and Communication Engineers of Japan on Jul. 20, 1981. The configuration of this type of surface acoustic wave device will be described as a prior art corresponding to the first invention with reference to FIG. 3. FIG. 3 is a schematic diagram of this prior art comprising three electrodes, which was drawn with reference to a photograph included in the above-described document. In FIG. 3, the surface acoustic wave filter of this prior art is structured such that two input transducers 2 and one output transducer 3 are arranged alternately on a substrate 1 for propagating a surface acoustic wave along the propagating direction of the surface acoustic wave, the input transducers 2 are connected to an input terminal 4 while the output transducer 3 is connected to an output terminal 5, and metal patterns 6 which are grounded are arranged outside the input transducers 2. To describe the prior art briefly, the number of electrodes and the number of electrode fingers are reduced to explain the basic structure of the device and the role of the metal patterns 6 outside the transducers is the same as that of the prior art.
FIG. 5 is a schematic diagram of a surface acoustic wave device proposed, for example, in Utility Model Application No.2-28125. In the figure, reference numeral 1 represents a parallelogrammic substrate for propagating a surface acoustic wave. On the substrate, two input transducers 2 and one output transducer 3 are arranged alternately along the propagating direction of the surface acoustic wave and the input transducers 2 are connected to the input terminal 4 while the output transducer 3 is connected to the output terminal 5.
A description is subsequently given of the operation of the above-described prior art.
Electric power of an input signal supplied to the input terminal 4 is divided by two and supplied to the input transducers 2 which convert the electric power into a surface acoustic wave. The converted surface acoustic wave propagates toward both sides of the input transducers 2 as shown by the arrows of FIG. 3 or FIG. 5. The output transducer 3 receives the surface acoustic wave and converts it into an electric signal. The converted electric signal is then output from the output terminal 5 as an output electric signal.
The surface acoustic wave propagating toward both end portions of the substrate 1 from the input transducers 2 is not received by the output transducer 3. Thereby, half of the total electric power being directed toward both ends of the substrate 1 for propagating the surface acoustic wave is discarded.
Generally speaking, when, at both end portions of the substrate 1 along the extension line of the propagation path of a surface acoustic wave, the surface acoustic wave leaked toward both end portions of the substrate 1 propagates from the input transducers constructed by metal patterns toward both end portions of the substrate 1 where the metal patterns do not exist, reflection is caused by mismatching of acoustic impedance due to the existence and non-existence of the metal patterns, leading to such deterioration of characteristics as ripples in amplitude characteristics and phase characteristics.
To overcome this problem, as shown in FIG. 3, metal patterns 6 are formed along the extension line of the propagation path of the surface acoustic wave. Then, mismatching of acoustic impedance is minimal or rarely occurs, thus causing little reflection, whereby the effect of reflection at both end portions of the substrate 1 of the surface acoustic wave leaked toward both end portions of the substrate 1 can be minimized.
Meanwhile, a surface acoustic wave device which makes use of irregular reflection at both end portions of the substrate 1 is shown in FIG. 5.
FIG. 27 is a plan view of a surface acoustic wave device of a prior art corresponding to the second invention as disclosed, for example, in Japanese Laid-Open Patent No.1-292908. FIG. 28 is also a plan view of a surface acoustic wave device provided with grating reflectors of another prior art as disclosed, for example, in Japanese Laid-Open Patent No.62-12206. FIG. 29 is also a plan view of a surface acoustic wave device provided with a multi-strip coupler of another prior art as disclosed, for example, in Japanese Laid-Open Patent No.62-31212.
A description is given of the construction of each of the above prior arts. Reference numeral 2 and 3 represent input and output inter-digital transducers (hereinafter abbreviated as IDT) for converting an electric signal into a surface acoustic wave or a surface acoustic wave into an electric signal, respectively. Reference numeral 7 represents grating reflectors for reflecting a surface acoustic wave, and 8 designates a multi-strip coupler (hereinafter abbreviated as MSC). Components 2, 3, 7 and 8 are arranged on the surface of the substrate 1.
The input and output IDTs are constructed by electrode fingers 2a and 2b and electrode fingers 3a and 3b, respectively, whose comb portions mesh with each other. The grating reflectors and the MSC are constructed by a plurality of strip lines.
Electron beam lithography may be used for patterning of the surface acoustic wave devices of these prior arts. In the electron beam lithography, an electron beam is irradiated onto a photosensitive agent for exposure unlike photolithography in which a photomask pattern is transferred by light. This electron beam lithography is very effective especially for the formation of fine patterns because of the short wavelength of a radiation source.
Electron beam exposure is applied in the production of a photomask used in the patterning of the above-described photolithography. The pattern configuration of a photomask is exactly the same as that of a surface acoustic wave device.
However, since the prior art surface acoustic wave device corresponding to the first invention is structured as described above, in the configuration of FIG. 3, the surface acoustic wave leaked from end portions of the input transducers 2 is hardly reflected at the boundaries of the metal pattern 6 and the input transducer 2. However, when the surface acoustic wave propagates from an end portion of the metal pattern 6 toward an end portion of the substrate 1, it is reflected at an end portion of the metal pattern 6 due to mismatching of acoustic impedance caused by the existence and non-existence of the metal pattern. This is based on the fact that the effect of reflection of the surface acoustic wave propagating from the input transducers 2 at the boundaries of the metal pattern 6 and the input transducer 2 can be reduced by adjusting the space between the input transducer 2 and the metal pattern 6, whereas it is difficult to control the effect at the boundaries of the metal pattern 6 and an end portion of the substrate 1. A surface acoustic wave device having 13 input and output transducers arranged alternately on the surface of a 64.degree. Y-rotation, X-propagation lithium niobate substrate like the above-described prior art was actually produced and it was found that the device was greatly affected by reflection, as shown in FIG. 4. The term, a 64.degree. Y-rotation, X-propagation lithium niobate substrate used herein, refers to a substrate over the surface of which a surface acoustic wave propagates and the plate normal of the surface of which is inclined at an angle of more than 59.degree. and less than 69.degree. from the Y axis of the crystalline axis toward the Z axis, and wherein the propagation direction of the surface acoustic wave forms an angle of more than -5.degree. and less than +5.degree. or more than 175.degree. and less than 185.degree. with respect to the X axis of the crystalline axis. This substrate has good performance in propagating a surface acoustic wave. FIGS. 4, 6, and 7 show frequency response characteristics in the pass band for prior art SAW filters. These are depicted in terms of loss as a function of frequency in the pass band. As shown on the vertical axis, loss decreases toward zero in the upward direction. The response of the prior art devices shows the presence of ripples in the frequency response characteristics in the pass band.
T1 of FIG. 4 represents a feedthrough signal (direct wave), t2 a main signal, t3 and t4 reflections from the metal pattern 6 of FIG. 3, and t5 reflection from an end portion of the substrate 1. Frequency characteristics resulting from these signals are shown in FIG. 6. Frequency characteristics obtained from the configuration of FIG. 5 are shown in FIG. 7. A number of ripples, although small, remain in both frequency characteristics.
The configuration of the prior art device shown in FIG. 5 in which the substrate 1 becomes bulky is not suitable to reduce the size of the device.
The first invention is intended to solve the above problems and it is an object of the present invention to provide a surface acoustic wave device which has a simple structure that can be constructed by the same method as that used for input or output transducers, can prevent deterioration of frequency characteristics caused by reflections of a surface acoustic wave leaked from both ends of the transducers, and can be made small in size.
since a conventional surface acoustic wave device corresponding to the second invention is structured as described above, in the case of patterning by means of the electron beam lithography, electrons irradiated from a radiation source onto a certain point of a photosensitive agent move in the agent and affect other parts of the photosensitive agent surrounding the point. As a result, photosensitive conditions may differ at each part of the agent and variations in line width called "proximity effect" may occur.
In other words, as shown in FIG. 30, root portions 2e and 3e of the electrode fingers may be thinner or partly broken, or, as shown in FIG. 31, the electrode fingers 2f and 3f at the uppermost end or having no patterns adjacent thereto may be thinner than other electrode fingers, or the width of the strip lines of the grating reflector and the MSC may be smaller than the width of other strip lines.
The photomask has similar problems.